Commercial necessity and governmental mandates have greatly increased the performance demands on the fuel systems of modern internal combustion engines. Such demands are especially rigorous for fuel systems used on compression ignition (diesel) engines. In particular, these engines must meet extremely challenging fuel efficiency and competitive cost objectives imposed by sophisticated commercial fleet and industrial users. They must also meet ever increasing emission control standards mandated by various governments around the world.
Among fuel system designers, there exists general agreement that increased emission standards will normally require operation at elevated injection pressure (e.g. above 15,000 to 18,000 psi). In addition, more accurate control over injection timing based on engine operating conditions will also be required. Meeting these demands is difficult enough from an engineering view point but is especially difficult in light of the commercial necessity of minimizing fuel system cost. Such cost is already a substantial part of the total manufacturing cost associated with most commercially available compression ignition engines.
One of the most successful fuel systems supplied for use on compression ignition engines has been the open nozzle, unit injector system pioneered by the Cummins Engine Company, Inc., assignee of the subject invention. This type of fuel system is characterized by a fuel injector including a cam driven reciprocating plunger and a nozzle containing injection orifices which remain open to the combustion chamber even when the reciprocating plunger is retracted to allow fuel to be metered into the injector prior to injection. This type of injector is characterized by greater simplicity as compared with injectors which employ a needle tip valve for closing the injector's orifices. An example of an early form of an open nozzle injector is illustrated in U.S. Pat. No. 4,280,659 to Gaal et al.
To increase the versatility of this type of injector and increase the fuel economy of an engine equipped therewith while improving the engine's performance, the single piece injector plunger has been replaced by a multi-element plunger assembly to form a variable volume timing chamber into which a controlled amount of incompressible liquid (such as fuel) can be metered and expelled on a cycle by cycle basis to vary the effective length of the injector plunger. See for example, FIGS. 16 and 17 of U.S. Pat. No. 3,951,117 to Julius Perr. Because the plunger reciprocation is controlled by a cam rotated in fixed synchronization with the engine's crank shaft, the timing of injection can be varied by varying the effective length of the injector's plunger assembly.
An important feature of the successful Cummins style open nozzle injector is its use of hydraulic control over both fuel metering and fuel timing. As discussed in much greater detail in U.S. Pat. No. 3,951,117, the amount of fuel metered into the metering chamber and the amount of timing fluid (such as an incompressible liquid, e.g. fuel) metered into the variable length timing chamber may be controlled by delivering the fuel and timing liquid through restricted orifices, respectively, and by varying the pressure of the supplied fuel or timing fluid to cause the amount of fuel/timing fluid, metered into the respective injector chambers, to be a function of pressure and the time available for metering. Such metering is known as pressure/time PT metering.
A number of additional patents have issued to the assignee of this invention, Cummins Engine Company, Inc., which are directed to open nozzle unit injectors having a timing plunger for forming a collapsible hydraulic link for varying the effective length of an injector plunger assembly to control the timing of injection on a cycle by cycle basis. See for example:
______________________________________ Patent No. Inventor ______________________________________ 4,986,472 Warlick, Timothy A. et al. 5,275,337 Kolarik, Oldrich S. et al. 5,299,738 Genter, David P. et al. 5,301,876 Swank, Bryan W. et al. 5,320,278 Kolarik, Oldrich S. et al. 5,323,964 Doszpoly, Bela et al. 5,445,323 Perr, Julius P et al. WO 97/06364 Peters, Lester L. et al. ______________________________________
While effective for the purposes intended, open nozzle unit injectors are subject inherently to unpredictable operational variations resulting from a variety of factors including the metering chamber of the injector remaining open during the period of injection plunger retraction. Ignition of the fuel/air mixture in the corresponding combustion chamber can cause combustion gases to be blown back into the fuel metering chamber of the injector thereby imparting variable pressure within the metering chamber and imparting varying pressure to the lowest plunger forming the injector's plunger assembly. This pressure variation can cause the amount of metered fuel/timing fluid to vary unpredictably in the subject injector or other injectors mounted in the same engine and supplied with fuel/timing fluid through common rails.
A variety of techniques have been employed in an attempt to ameliorate the problems associated with undesired variation in the supply pressure in fuel injection systems using open nozzle injectors wherein the fuel and/or timing fluid is metered based on variation in the supply pressure. For example, a check value may be placed in the supply rail such as illustrated at 522 and 533 of FIG. 16 of the Perr '117 patent. Note also U.S. Pat. No. 5,611,317 which discloses a check valve adjacent the fuel metering chamber for limiting the effect of pressure variations in the fuel metering chamber of the injector.
As disclosed in U.S. Pat. No. 5,037,031, open nozzle injectors are also prone to malfunction due to carboning of fuel in the injector. This patent discloses a technique for minimizing the effect of carboning of open nozzle injectors by use of a labyrinth flow area formed by a specially designed cup bore and stepped inner plunger. U.S. Pat. No. 5,209,403 discloses scavenging flow to remove blow back gases and to cool the injector and also discloses the use of check valve 57, col. 7, lines 25+. See also check valve 46 of U.S. Pat. No. 5,445,323 for use in the scavenging flow path of a open nozzle fuel injector.
While effective for the purposes intended, undesired variation in the timing and metering of fuel injection in open nozzle unit injectors may still occur. For example, the combustion of fuel within one combustion chamber of a multi-cylinder internal combustion engine equipped with open nozzle unit injectors having hydraulically variable timing as described above can have the effect of creating pressure pulses in the supply, drain or timing lines (rails) leading to adjacent unit injectors. These pressure pulses are only partially diminished by the check valves and plunger positioning used in the references disclosed above to prevent undesirable pressure variation in the rail lines supplying and draining the various injectors.
In other types of fuel injection systems, such as unit injectors having closed nozzles as disclosed in U.S. Pat. Nos. 4,976,244 and 4,951,631 or such as pump/distributor systems as disclosed in U.S. Pat. No. 4,557,240, timing plungers are used to provide variation in injection timing. Some of these timing plungers include radial flanges for positively stopping the corresponding timing plunger when the plunger flange engages a stop surface. However, such teachings to not suggest how to avoid the effects of pressure variations occurring in open nozzle unit injectors equipped with variable hydraulic timing.
A need therefor exists for an open nozzle, unit injector which employees variable hydraulic timing but overcomes the deficiencies of the prior art as discussed above. In particular, a need exists for such an injector which is less susceptible to unpredictable timing variations.